This is one of the coins I brought for a birthday present. You can see the sales image here https://www.ma-shops.com/mueller/item.php?lang=en&id=24359&ref=favBew I didn't notice the light blue color on the plalette of the coin, but I see it clear on the images i took of the coin. It looks like some corrosion. I didn't think so at first. But after thorough examination, I don't know what else it can possibly be.
Pretty straightforward. If that's PVC residue, soaking in acetone should remove it. Let's see what others think.
This awesome coin has appeared on this forum a number of times recently. I'll share the card that came with mine again. It provides a guide to the symbols on the coin.